Measurement results

As SRM has an identical geometrical structure, only one phase can be used for measurement, and only half electric period of 30° mechanical degrees are adopted for measurement. The remaining part of characteristics can be estimated by proper

Figure 12.

The wiring diagram for the measurement platform.

L i,ð θÞ ¼λði,θÞ=di (13) 5. Experimental measurement methods

Generally, the experimental measurements can be categorized into direct and indirect methods [21, 24]. The direct methods utilize magnetic sensors to directly measure pole flux [24], or they may measure the pole flux directly after proper processing of induced voltage over search-coil that is mounted on stator pole [14].

Figure 11.

The FEM-calculated characteristics for the studied 8/6 SRM. (a) Flux linkage for 8/6 SRM. (b) Inductance for 8/6 SRM. (c) Torque for 8/6 SRM.

The direct methods are rarely used because of the leakage flux that affects the accuracy [24]. On the other hand, indirect methods uses phase voltage and current to estimate flux [12]. They can provide simple structure, low cost, and better accuracy [24]. Hence, they are adopted in this work.

5.1 Measuring method and platform

The phase voltage (V) and current (i) are used to measure phase flux linkage indirectly. At a desired rotor position (θ), a pulsed dc voltage is applied to one phase winding, and the phase voltage and current are measured and recorded. The flux-linkage (λ) is calculated according to Eq. (10) or its discrete form in Eq. (11) [14].

λði,θÞ ¼ ð

V�R�i

ð Þ �dtþλð Þ0 (14)

λð Þ ¼n Xⁿ

k¼1

V kð Þ �R�i kð Þ

½ � �Tsþλð Þ0 (15)

where R, n, Ts are the phase resistance, adopted number of samples and sam-pling period, respectively.λ(0) is the initial pole flux. It equals to zero because SRM has no magnets.

The same principle is used for torque measurement, but the phase current and torque signals are measured and recorded directly while rotor is locked at a specific position. The measurement procedure of flux/torque should be repeated several times owing to the desired angle resolution to generate the complete flux/torque data. Figure 12 shows the schematic diagram of measurement platform.

5.2 Measurement results

As SRM has an identical geometrical structure, only one phase can be used for measurement, and only half electric period of 30° mechanical degrees are adopted for measurement. The remaining part of characteristics can be estimated by proper

Figure 12.

The wiring diagram for the measurement platform.

mirroring. The measurement process starts at the unaligned position (θ= 0°) and ends at the aligned position (θ= 30°).

5.2.1 The measured results of flux-linkage

The tested 8/6 SRM is equipped with a search coil on stator poles. The search coil is used for verification purposes. The integration of voltage (e) induced on search coil gives directly the phase flux linkage. The measured phase voltage and current

Figure 13.

The measured waveforms at 17°for (a) voltage and current, (b) flux.

Figure 14.

The measured and FEM flux curves.

are given in Figure 13(a). Their corresponding measured flux linkage is illustrated in Figure 13(b). The adopted current for measurements is 20A with positioning step of 1°. Figure 14 shows the obtained flux curve at five different rotor positions.

It compares between the direct, indirect, and FEA methods. A very good agreement can be seen.

5.2.2 The measured results of static torque

The electromagnetic torque of the SRM is measured directly using a DRBK torque sensor. The DRBK has a limited sampling frequency of 1 kHz. This may affect the measurement accuracy. Better accuracy can be achieved if it is possible to measure reasonable number of samples. This can be achieved by increasing the measurement time. This time is the rising time of phase current. Adding an external inductance in series with phase winding can extend measurement time.

The added inductance can increase time constant. Hence, current increase becomes slower allowing recording more torque samples. Figure 15 shows the measured torque signal along with phase current at position of 16.5°. The measured torque curves are shown in Figure 16. Figure 17 shows a comparison between measured and FEM-calculated torque characteristics, a very good agreement is observed.

Figure 15.

The measured current and torque waveforms at 16.5°.

Figure 16.

The measured torque curves.

mirroring. The measurement process starts at the unaligned position (θ= 0°) and ends at the aligned position (θ= 30°).

5.2.1 The measured results of flux-linkage

The tested 8/6 SRM is equipped with a search coil on stator poles. The search coil is used for verification purposes. The integration of voltage (e) induced on search coil gives directly the phase flux linkage. The measured phase voltage and current

Figure 13.

The measured waveforms at 17°for (a) voltage and current, (b) flux.

Figure 14.

The measured and FEM flux curves.

are given in Figure 13(a). Their corresponding measured flux linkage is illustrated in Figure 13(b). The adopted current for measurements is 20A with positioning step of 1°. Figure 14 shows the obtained flux curve at five different rotor positions.

It compares between the direct, indirect, and FEA methods. A very good agreement can be seen.

5.2.2 The measured results of static torque

The electromagnetic torque of the SRM is measured directly using a DRBK torque sensor. The DRBK has a limited sampling frequency of 1 kHz. This may affect the measurement accuracy. Better accuracy can be achieved if it is possible to measure reasonable number of samples. This can be achieved by increasing the measurement time. This time is the rising time of phase current. Adding an external inductance in series with phase winding can extend measurement time.

The added inductance can increase time constant. Hence, current increase becomes slower allowing recording more torque samples. Figure 15 shows the measured torque signal along with phase current at position of 16.5°. The measured torque curves are shown in Figure 16. Figure 17 shows a comparison between measured and FEM-calculated torque characteristics, a very good agreement is observed.

Figure 15.

The measured current and torque waveforms at 16.5°.

Figure 16.

The measured torque curves.